Communications and radar industries have an interest in microwave transmission coaxial cable systems where low-frequency and/or large bandwidth operation makes the use of rectangular or ridged waveguide impractical. Lengths of air-filled rigid coaxial cable are typically joined together using commercially available RF connectors which feature a center conductor support typically made, in part, of poly-tetrafluoroethylene (PTFE) or Teflon™ at the connector interface. This thermoplastic material is most often favored because its material properties include low dielectric loss, negligible water absorption, chemical inertness, and a −200° C. to +250° C. useful temperature range. However, Teflon's thermal conductivity is very poor and it cold flows under the combined effects of mechanical stress and temperature. Consequently, center conductor temperature rise due to conductor and dielectric dissipative losses severely limits the average amount of microwave power allowable on rigid cable using PTFE dielectric. This is because, for long lengths of rigid air-filled coax cable, the center conductor is cooled mainly by gaseous convective cooling within the cable interior, and less so by radial cooling through the dielectric supports at the RF connector interfaces. An example from the Andrew Broadcasting Catalogue 35 is their Type 561 Standard Rigid Coaxial Transmission Line (50 ohms) with EIA 1⅝ standard coupling flanges using inner connector Model 34660 with PTFE dielectric. At 800 MHz, inner connector at 102° C. (216° F.), ambient temperature 40° C. (104° F.), filled with dry air: this cable system is rated for only 5.19 kilowatts average power.
Another example of average power limitation due to the need to limit coax cable center conductor temperature is in the high-power klystron, klystrode, and broadband travelling wave tube (TWT) industry. Coaxial ceramic disk windows are sometimes used at the output port of these high-power amplifiers. The brazed-in ceramic maintains vacuum-tight integrity between ambient air pressure and the amplifier tube's evacuated interior, yet allows the passage of microwave power into the customer's coaxial transmission line. For high average power applications, one source of window ceramic failure occurs if too high a temperature gradient exists between the ceramic inner diameter and its outer perimeter. Stress cracking occurs in the ceramic window if the radial thermal gradient is too high.
Cooling a coax cable center conductor is also desirable for low-loss transmission of microwave signals. In this application, imagine a set of ground-based microwave antennas that collect a faint signal from a distant space probe, which must be communicated with as little loss as possible through respective phase length-matched coax cable transmission lines to a reactive-type power combiner.